This invention relates to photoconductive materials, and more particularly, to their use in electrophotography.
It is known that images may be formed and developed on the surface of certain photoconductive materials by electrostatic means. The basic xerographic process, as taught by Carlson in U.S. Pat. No. 2,297,691, involves uniformly charging a photoconductive insulating layer and then exposing the layer to a light-and-shadow image which dissipates the charge on the areas of the layer which are exposed to light. The electrostatic latent image formed on the layer corresponds to the configuration of the light-and-shadow image. This image is rendered visible by depositing on the image layer a finely divided developing material comprising an electroscopic marking material called a toner. The powder developing material will normally be attracted to those portions of the layer which retain a charge, thereby forming a powder image corresponding to the latent electrostatic image. This powder image may be transferred to paper or other receiving surface. The paper then will bear the powder image which may subsequently be made permanent by heating or other suitable fixing means. The above general process is also described in U.S. Pat. Nos. 2,357,809; 2,891,011 and 3,079,342.
That various photoconductive insulating materials may be used in making electrophotographic plates is known. Suitable photoconductive insulating materials such as anthracene, sulfur, selenium or mixtures thereof, have been disclosed by Carlson in U.S. Pat. No. 2,297,691. These materials generally have sensitivity in the blue or near ultra-violet range, and all but selenium have a further limitation of being only slightly light sensitive. For this reason, selenium has been the most commercially accepted material for use in electrophotographic plates. Vitreous selenium however, while desirable in most aspects, suffers from some limitations in that its spectral response is somewhat limited to the ultraviolet, blue and green regions of the spectrum and the preparation of vitreous selenium plates requires costly and complex procedures, such as vacuum evaporation. Also, selenium plates require the use of a separate conductive substrate layer, preferably with an additional barrier layer deposited thereon before description of the selenium photoconductor. Because of these economic and commercial considerations, there have been many recent efforts towards developing photoconductive insulating materials other than selenium for use in electrophotographic plates.
It has been proposed that various two-component materials be used in photoconductive insulating layers used in electrophotographic plates. For example, the use of inorganic photoconductive pigments dispersed in suitable binder materials to form photoconductive insulating layers is known. It has further been demonstrated that organic photoconductive dyes and a wide variety of polycyclic compounds may be used together with suitable resin materials to form photoconductive insulating layers useful in binder-type plates. In each of these two systems, it is necessary that at least one original component used to prepare the photoconductive insulating layer be, itself, a photoconductive material.
In a third type plate, inherently photoconductive polymers are used; frequently in combination with sensitizing dyes or Lewis acids to form photoconductive insulating layers. Again, in these plates at least one photoconductive component is necessary in the formation of the layer. While the concept of sensitizing photoconductors is, itself, commercially useful, it does have the drawback of being limited to only those materials already having substantial photoconductivity.
The above discussed three types of known plates are further described in U.S. Pat. Nos. 3,097,095; 3,113,022; 3,041,165; 3,126,281; 3,073,861; 3,072,479; 2,999,750; Canadian Patent No. 644,167 and German Patent No. 1,068,115.
The polymeric and binder-type organic photoconductor plates of the prior art generally have the inherent disadvantages of high cost of manufacture, brittleness, and poor adhesion to supporting substrates. A number of these photoconductive insulating layers have low temperature distortion properties which make them undesirable in an automatic electrophotographic apparatus which often includes powerful lamps and thermal fusing devices which tend to heat the xerographic plate. Also, the choice of physical properties has been limited by the necessity of using only inherently photoconductive materials.
Inorganic pigment-binder plates are limited in usefulness because thay are often opaque and are thus limited to use in systems where light transmission is not required. Inorganic pigment-binder plates have the further disadvantage of being non-reusable due to high fatigue and rough surfaces which make cleaning difficult. Stil another disadvantage is that the materials used have been limited to those having inherent photoconductive insulating properties.
It has recently been discovered that certain aromatic polymers having slight photoconductivity may be sensitized by suitable Lewis acids to produce photoconductive insulating layers having good physical properties and satisfactory sensitivity for many electrophotographic applications. Typical polymers which may be sensitized are: poly-N-vinyl carbazole, aromatic phenolic resins as described in U.S. Pat. Nos. 3,408,183; 3,408,186; 3,408,188; 3,408,190 and others. These resins may be sensitized with any of a number of non-photoconductive Lewis acids to produce useful electrophotographic plates. While these plates are useful for many purposes, they tend to have low sensitivity with most Lewis acids and often have a color cast characteristic of the Lewis acid or of the complex formed by the Lewis acid and the aromatic resin. Where an image is to be developed directly on the photoconductive layer and the imaged sheet is to be used as a transparency for projection, the color cast is objectionable. Also, many of these plates have low image resolution characteristics, presumably due to objectionably high lateral conductivity. Thus, there is a continuing need for improved Lewis acids to produce electrophotographic plates with the aromatic resins which are known to form charge transfer complexes.